Irrigation is typically used to water large, homogeneous areas such as fields, lawns, and gardens. The water is assumed to be available from a single source, such as a well, canal, or municipal water system. Water from municipal water systems are often stressed during times of heat and drought, and watering restrictions are frequently implemented to provide adequate water for higher priority uses. These restrictions may start as odd-even day lawn watering and progress to complete bans on lawn watering, and finally to complete bans on garden watering.
The fields and lawns typically have a single species of plant and water application is based on water sensors, evapotranspiration models, or rules. This type of irrigation system can be inadequate for yards and gardens where numerous species are growing in close proximity, particularly large water users like trees and shrubs in proximity to lesser water users. Trees, shrubs, and structures also provide shade, which impacts evapotranspiration, which is the sum of evaporation and plant transpiration.